This invention relates to economical and efficient process for making isocyanatosilanes.
Currently, all of the known methods for isocyanatosilanes production suffer from byproduct formation. The economics of isocyanatosilanes are highly dependant on the yield of product, thus, a process whereby the isocyanatosilane can be cleanly produced is highly sought. We have invented a process where the carbamatosilane is prepared in such a way that the byproduct salt is inert in nature and can be carried thru the cracking apparatus without generating heavy byproducts. In the past, the one preferred method of preparing acceptable carbamatosilane was to distill the feedstock prior to cracking. If the distillation was completed prior to cracking, impurities that could be detrimental to the cracking reaction were removed from the carbamate. With this invention, it can be shown that when a pot profile of the reaction mixture is analyzed during the cracking reaction, the profile of distilled and non-distilled are similar in appearance and the isocyanate lifetime is much improved from previously used non-distilled methods. With the improved lifetime of isocyanate, isolation of the isocyanate from the reactive zone will be easier to accomplish, and thus, the overall process will approach the performance observed with distilled feed.
A variety of processes are known for making isocyanatosilanes. Commercial synthesis of isocyanatosilanes is best demonstrated by the thermal cracking of the corresponding carbamatosilanes. In this process, the carbamate is subjected to high temperature under subatmospheric pressure. The patent literature teaches 3 basic procedures that involve thermal decomposition of carbamates:
1) cacking the neat carbamate;
2) utilization of inert media to assist in the cracking; and
3) vapor phase cracking using a hot tube cracker.
Thermal decomposition is the preferred method because it avoids the use of highly toxic and environmentally destructive phosgene. Due to the high reactivity of the isocyanate, all of these methods require that both the isocyanate and alcohol are quickly removed from the high temperature zone and then separated to prevent back addition of the alcohol. Although the isocyanate and alcohol separation can be routinely done by continuous fractional distillation while cracking, removing the reactive isocyanate from the cracking zone proves a challenge. Any isocyanate remaining in contact with the thermal medium will be further transformed into a range of non-desirable materials.
More recently, U.S. Pat. No. 6,388,117 teaches a continuous process that involves the addition of a tin catalyst prior to cracking in a cleavage and rectification unit. In this patent a portion of the material is routinely purged from the bottom of the cleavage reactor. Purging effectively serves two purposes: first, to maintain constant catalyst load; and second, to keep high molecular weight components at a constant level. The amount preferably removed from the bottom of the reactor is 15–25 percent by weight. This material is then allowed to mix with alcohol to quench the isocyanate, redistilled and partially fed back into the cracking zone.
U.S. Pat. Nos. 5,886,205 and 6,008,396 both teach a process where an inert solvent is used. These patents teach very similar methods except the former claims that pH control is required along with a transition metal catalyst. By utilizing the inert solvent, a large amount of waste is possible unless the high molecular weight material can easily be removed from the inert solvent. The economics would limit this type of separation.
High temperature vapor phase process is described in U.S. Pat. No. 5,393,910, DE 10064086 (U.S. Pat. Application Pub. No. 2004/0049064) and DE 10325608 (U.S. Pat. Application Pub. No. 2004/0249179). This vapor phase process suffers from the requirement of specialized equipment capable of high temperature operation, with the concurrent extensive capital investment. It is also reported in non-silyl isocyanate patents that coking of the reactor has hindered the commercialization of such units.
DE 10161272 describes a method where the carbamate is cracked in the presence of a high molecular weight isocyanate and transition metal catalyst. This process would likely suffer from heavies (i.e. nonvaporizable materials) isolation as the inert solvent patents described above do.
JP 9328489 teaches a method where the 3-aminopropylsilane is reacted first with isocyanate such as MDI to give the corresponding urea, which was then thermally cracked using catalytic conditions.
Other procedures are described that utilize a low temperature cracking of a carbamate derivative. For example, U.S. Pat. No. 4,697,009 describes a process where an acyl-urea group is utilized as the leaving group rather than alkyl alcohols that are most common. This process suffers from the intermediate preparation that involves difficult separation of solvent and the resulting salt.
U.S. Pat. No. 4,064,151 discloses the preparation of isocyanates by preparing halosilyl carbamates by direct reaction of aminosilane in the presence of CO2 and halosilyl compounds, and a tertiary amine acid scavenger. The resulting halosilyl carbamate decomposes at a relatively low temperature to yield the isocyanate. However, a difficult workup is required to obtain the product of this process.
DE 10108543 describes a process where the carbamatosilane is reacted directly with methyl trichlorosilane to give the N-silylated carbamate, which then decomposes under slight heating to give the isocyanate and an equimolar amount of alkoxychloromethylsilane. This method suffers from the requirement of an acid trap such as triethylamine, which then requires separation and disposal or recycle.
Typical of non-cracking methods are found in JP 09208589 and U.S. Pat. No. 4,654,428 in which the aminopropylsilane is directly reacted with highly toxic phosgene to yield the desired isocyanate.
In the current literature, the starting carbamatosilane is prepared by multiple processes. U.S. Pat. No. 6,388,117 teaches the preparation of carbamatoorganosilane by the reaction of aminoorganosilane, urea, and alcohol to form the carbamatoorganosilane. This distillation is claimed to be an important part of the process to obtain carbamatoorganosilanes and the corresponding isocyanatosilane.
U.S. Pat. No. 5,218,133 describes the preferred synthesis of carbamatosilane by reacting aminosilane with dialkylcarbonates in the presence of a strong base such as sodium methoxide. The residual sodium methoxide is then neutralized with a carboxylic acid. Although this carbamatosilane is then used to prepare silylisocyanurates in the presence of transition metal catalysts, U.S. Pat. No. 5,393,910 describes this method to be the preferred for the preparation of carbamatosilanes used in the manufacture of isocyanatosilanes via gas phase pyrolysis. No mention of distillation is noted in this patent.